Microsoft's long-awaited release of the upgrade to its flagship Windows operating system will likely be delayed again by at least three months, research group Gartner said on Tuesday. The research note, released to clients on Monday, said the new Windows Vista operating system is too complex to be able to meet Microsoft's targeted November release for volume licence customers and January launch for retail consumers. A Microsoft spokeswoman said the company disagreed with the Gartner report and it was still on track to meet its launch dates.